In this paper, we explain the HNO3 observations made with the MIPAS/Envisat instrument in the northern polar region at the time of the October-November 2003 solar proton event. Increases of 0.5–5.5 ppbv are seen at altitudes 35–58 km after the onset of the event. Results from the Sodankylä Ion and Neutral Chemistry model are in good agreement with the MIPAS observations, except at around 58 km where the modeled mixing ratios are about a factor of two larger. According to the model results, HNO3 production at altitudes above 35 km is almost entirely due to ion-ion recombination between NO3− and H+ cluster ions. At 35 km and below, there is contribution also from N2O5 reacting with ions as well as from NO2 reacting with OH.